It is known that a vehicle is provided with an air-fuel ratio sensor that detects an air-fuel ratio of exhaust gas. The air-fuel ratio sensor is disposed upstream of a catalyst for purifying the exhaust gas. The signal detected by the air-fuel ratio senor is feed backed to enhance a purifying efficiency of the catalyst. In order to prevent a deterioration of the air-fuel ratio sensor, diagnosis of the air-fuel ratio sensor is conducted.
U.S. Pat. No. 5,724,952 shows such a diagnosis system in which a PI feedback control is conducted to periodically inverse a detected equivalence ratio of the air-fuel ratio sensor. The inversion period is compared with a degradation determining value to determine whether a response degradation of the air-fuel ratio sensor exists.
The response degradation of the air-fuel ratio sensor includes a time delay degradation and a time constant degradation. An exhaust emission is differently affected by the time delay degradation and the time constant degradation. However, in the diagnosis system shown in U.S. Pat. No. 5,724,952, the time delay degradation and the time constant degradation are not distinguished from each other to conduct the degradation diagnosis of the air-fuel ratio sensor. The degradation diagnosis is conducted based on the inversion period which varies with effects of the time delay degradation and the time constant degradation. Thus, the degradation of the air-fuel ratio sensor is hardly diagnosed accurately.